vmdk: support vmfsSparse files

vmdk: support vmfsSparse files

VMware ESX hosts use a variant of the VMDK3 format, identified by the
vmfsSparse create type ad the VMFSSPARSE extent type.

It has 16 KB grain tables (L2) and a variable-size grain directory (L1).
In addition, the grain size is always 512, but that is not a problem
because it is included in the header.

The format of the extents is documented in the VMDK spec.  The format
of the descriptor file is not documented precisely, but it can be
found at http://kb.vmware.com/kb/10026353 (Recreating a missing virtual
machine disk (VMDK) descriptor file for delta disks).

With these patches, vmfsSparse files only work if opened through the
descriptor file.  Data files without descriptor files, as far as I
could understand, are not supported by ESX.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
--
v2: Rebase to patch 01.
    Change le64_to_cpu to le32_to_cpu.
    Rename vmdk_open_vmdk3 to vmdk_open_vmfs_sparse, which represents the
    current usage of this format.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>